PROJECT SUMMARY UBE3A is an E3 ubiquitin ligase that targets itself and other substrates for proteasomal degradation. In the developing brain, neuronal progenitors and immature neurons biallelically express Ube3a, but as neurons mature, Ube3a expression becomes restricted to the maternally-inherited allele. Mutations that elevate maternal or paternal Ube3a are linked to autism risk, but precisely how UBE3A excess impairs neurodevelopment is unclear. Recently, we found that phosphorylation of threonine 485 (T485) inhibits UBE3A ubiquitin ligase activity. UBE3A T485 phosphorylation initiates embryonically and peaks at birth, suggesting that phosphorylation might protectively limit UBE3A activity during early cortical development. Additionally, we found that an autism-linked de novo mutation in UBE3A (T485A) disrupts this phosphorylation site, effectively locking UBE3A always-on. We engineered a mouse that precisely models this human UBE3A T485A mutation, allowing us to evaluate how this novel gain-of-function mutation affects brain and behavioral phenotypes when inherited maternally or paternally. In preliminary studies, we found that cortical thickness and brain weight were significantly increased at birth in all three Ube3a T485A genotypes (paternal, maternal, homozygous). Mutations in other autism-linked genes increase brain weight to a similar extent. These findings suggest a novel and previously unrecognized prenatal function for UBE3A in brain development. All three Ube3a T485A mutant genotypes also had behavioral phenotypes consistent with neurodevelopmental disorders. Since little is known about how UBE3A impairs brain function at any age, we performed unbiased proteomics to identify brain-relevant substrates. Our preliminary proteomics data link UBE3A directly to the proteasome, a structure that can influence the cell cycle and signaling pathways important for brain development. These and other data lead us to hypothesize that UBE3A T485A alters the balance of cell proliferation and differentiation during brain development, in part by impairing proteasome function, and contributes to autism-associated phenotypes later in life. The experiments in this proposal will rigorously demonstrate that (1) UBE3A T485A alters the balance of progenitor proliferation and differentiation in the cerebral cortex, (2) parent-of-origin inheritance of Ube3a T485A influences autism-related brain and behavioral phenotypes, and (3) UBE3A T485A interacts with the proteasome and impairs proteasome function in the brain. Unbiased proteomics experiments will identify brain-relevant substrates of UBE3A, and broaden our understanding of which molecular pathways are affected by gain-of-function mutations that enhance UBE3A activity.